Variation in Near-Infrared Spectra of Water Containing Polyhydric Alcohol

Abstract

Near-infrared absorption spectra of aqueous solutions of eleven polyhydric alcohols (nine dihydric alcohols and two trihydric alcohols) at concentrations up to 20% were obtained at 20, 25, and 30 °C. Variations in the band due to O–H stretching vibration overtones of water and alcohol with changes in concentration and temperature were determined by a multivariate curve resolution-alternating least squares analysis, which identified the components of the band causing the spectral variation. The band consisted of three common components almost independent of the alcohol type. The first and the second components are attributed, respectively, to water molecules weakly hydrogen-bonded (or non hydrogen-bonded) and those strongly hydrogen-bonded with other water molecules, while the third component are due to water interacting with the alcohol and to the alcohol itself. The abundance of the first and third components decreased and increased, respectively, as the alcohol concentration increased. In contrast, the abundance of the second component increased initially and then decreased. The initial increase corresponds to the enhancement of hydrogen bonding by hydrophobic interactions. The subsequent decrease is due to an increase in water–alcohol interactions and a decrease in water concentration. The maximum increase in the abundance of the second component depended on the type of alcohol. The increase in abundance was greater for alcohols with larger alkyl groups. In contrast, the increase in abundance of the second component was smaller for alcohols with more hydric groups.